The present invention relates to an exposure method and apparatus for projecting an image of a pattern that has been formed on a mask, through projection optics, onto a photosensitive substrate. More particularly, the present invention relates to a method and apparatus for exposing a mask pattern on a substrate in a good or properly focused imaging state.
Recent demands for higher levels of integration in semiconductor devices have created corresponding requirements for higher resolution in projection exposure equipment. This has resulted in the use of projection optics with greater (larger-numbered) numerical apertures. In general, in an optical system, the depth of focus of the image is inversely proportional to the square of the numerical aperture. In other words, increasing the numerical aperture makes it harder to keep shot areas on the substrate within the (reduced) depth of focus of the optics. In the past, there have been auto-focus mechanisms that can measure the position of the exposure surface of the photosensitive substrate to detect the positional displacement along the optical axis of the optical system between the exposure surface of the substrate and the actual image formation plane of the projection optics, at prescribed measurement points within the shot areas of the photosensitive substrate. The position of the photosensitive substrate in the optical axis direction of the optical system is then adjusted to keep this displacement within a prescribed tolerance (the depth of focus).
Projection optical systems generally have "image curvature aberration," resulting in positional offset between off-axis and on-axis portions of the formed image. Therefore, in order to have high-resolution exposure over the entire shot area on the photosensitive substrate surface, the position of the substrate along the optical axis of the projection optics must be controlled in a manner that takes the image curvature aberration of the optical system into account. In other words, a permissible range of positions must be determined along the optical axis within which the surface of the shot area will be within the depth of focus of the optical system. The exposure can then be performed with the photosensitive substrate positioned within that range.
In this type of projection exposure system, the entire mask is commonly expose-transferred onto each shot area of the photosensitive substrate, one by one. The exposure can also be performed, however, with portions of the mask blocked out so that only a prescribed or selected area on the mask is transferred to the photosensitive substrate. In this mode as well, the position of the photosensitive substrate along the optical axis of the optical system is adjusted by a method similar to that used when the entire mask is exposed. In other words, the permissible range of positions along the optical axis of the projection optics must be determined for each shot area on the photosensitive substrate, so as to ensure that the entire shot area surface will be kept within the depth of focus of the projection optical system.